T cells (green) and B cells (red) fight invaders, but
when not needed must be kept in check to prevent them from attacking the
body’s own tissues and causing autoimmune disorders. Two “leash”
proteins and a communication protein appear to help keep these trigger-happy
cells from erupting into friendly fire.

Measuring messenger RNA, which acts like an order slip
for building a copy of a gene’s protein, gives scientists a feel
for a gene’s activity level in a cell. This in turn allowed Peng’s
group to highlight genes with distinct differences in activity levels
in the mice with lupus-like symptoms.

In addition to their work with immune cell leashes, Peng
and his colleagues recently connected lupus in mice to a protein that
is involved in immune system communications.

SCIENTISTS ARE UNCOVERING NEW
COMPLEXITIES in the innermost workings of the human immune system
that could make big differences for patients with autoimmune diseases.
Thousands of Americans are diagnosed with these disorders each year as
cells in their bodies that normally attack invaders like bacteria and
viruses instead turn their fury on the body’s own tissues. This
about-face causes conditions such as lupus, myasthenia gravis, allergies,
psoriasis, diabetes, Graves’ disease, rheumatoid arthritis and multiple
sclerosis.

For decades, scientists assumed that these disorders were
caused mostly by bad instructions to the cells that serve as the immune
system’s attack dogs. These cells, which are collectively referred
to as lymphocytes and include B cells and T cells, rely on a complex signaling
and detection system that tells them when, where and what to attack. If
immune attack cells were assaulting the wrong targets, researchers reasoned,
something had to be going awry in that signaling system.

Thanks to the work of researchers like Stanford Peng, MD,
PhD, assistant professor of medicine in rheumatology and of pathology
and immunology, a much more complicated picture of the causes of autoimmune
diseases is beginning to emerge. Expanded insights into these causes may
soon be offering scientists new frontiers for developing drugs that can
ease or prevent such disorders.

One of the biggest new developments in autoimmune theory
focuses on what immune attack cells are like when they’re not on
the job battling invaders. Scientists previously assumed that mature,
unused versions of T and B cells were “sleeping” or dormant.

But a new theory starting to gain widespread acceptance
suggests that the cells are constantly spoiling for a fight, and healthy
immune systems have to constantly work to restrain them, in effect putting
a “leash” on the attack dogs.

In both T and B cells, Peng has identified the first-ever
examples of these leashes — proteins that actively work within the
cells to keep them quiet when they’re not needed.

Peng specializes in the study of lupus, an autoimmune condition
that afflicts approximately 1.5 million Americans with a range of symptoms
including arthritis, prolonged fatigue, skin rashes, kidney damage, anemia
and breathing pain.

Through selective breeding, scientists have developed several
mouse models that exhibit one or more lupus-like symptoms. To identify
the gene leashes, Peng’s research group compared levels of messenger
RNA for various genes in normal mice and a lupus mouse model.

Measuring messenger RNA, which acts like an order slip for
building a copy of a gene’s protein, gives scientists a feel for
a gene’s activity level in a cell. This in turn allowed Peng’s
group to highlight genes with distinct differences in activity levels
in the mice with lupus-like symptoms.

The first leash they found, a protein called Foxj1, had
never previously been linked to immune system functions. Based on messenger
RNA levels, though, the gene appeared to be much less active in lupus
mice than in normal mice. When Peng and colleagues disabled the gene for
the protein in normal mice, the mice developed lupus-like symptoms.

“These symptoms included inflammation in multiple
organs like their lungs, their salivary glands, their kidneys, and other
organs, which is very characteristic of lupus,” Peng explains.

Scientists had previously identified Foxj1 as a transcription
factor, a protein that can bind to DNA to increase or decrease the activity
of other genes. Further investigation by Peng’s group showed that
decreased Foxj1 activity led another transcription factor, NF-κB, to increase
its activity.

“This protein belongs to a family of transcription
factors heavily implicated in various types of inflammation, including
the inflammation caused by infections and by allergies,” Peng says.
“So our thinking is that without Foxj1, more NF-κB is activated,
possibly triggering the inappropriate activation of T cells and leading
to organ inflammation and other lupus symptoms.”

Inappropriately activated T cells also are involved in multiple
sclerosis and in diabetes, suggesting that Foxj1 also might be a contributing
factor in these conditions, Peng notes.

The second leash recently identified by Peng’s group
is known as microphthalmia-associated transcription factor (MITF). Microphthalmia
is a genetic condition that causes abnormally small eyes and impaired
vision.

Like the Foxj1 protein, Peng’s group became interested
in MITF when messenger RNA studies suggested the gene was unusually inactive
in a mouse lupus model. Peng and colleagues lowered activity levels of
the protein in normal mice, and close examination of those mice showed
that B cells were spontaneously turning themselves on and making antibodies,
clumps of proteins that are normally designed to attack invaders. The
new antibodies in the mice were autoantibodies — antibodies targeted
to the body’s own tissues that are a characteristic symptom of lupus.

“This is the first transcription factor we’ve
found that has to be active in the resting B cell to keep it that way,”
Peng says.

MITF’s sphere of influence is proving a little harder
to define than that of Foxj1. It appears to restrain interferon regulatory
factor 4 (IRF4), a transcription factor previously linked to the activation
of B cells. But it appears to have that effect by proxy through its influence
on several other genes that in turn act to keep IRF4 in check.

“We’ve been focusing our efforts to develop
new treatments for autoimmune disease on pathological targets —
genes that are overused or are used inappropriately, leading to immune
system attacks on self,” Peng says. “Another concept we should
keep in mind is that the loss of one of these regulatory genes that keep
the immune system in check also may be a primary contributing factor.”

In addition to their work with immune cell leashes, Peng
and his colleagues recently connected lupus in mice to a protein that
is involved in immune system communications.

The protein, SLAM-associated protein (SAP) appears to be
involved in exchanges between B cells and T cells. Scientists have long
known that T cells “talk” to B cells to help them produce
antibodies meticulously customized to destroy the last scattered remnants
of a persistent invader. But they’ve had a hard time determining
the details of how those interactions take place.

“SAP may give us an important first insight into how
this occurs,” Peng says. “But even more importantly, it may
provide us with a target for new lupus treatments that don’t widely
suppress the immune system.”

Earlier research had shown that higher levels of SAP were
present in animals with autoimmune conditions than in normal animals.

Peng affirmed the SAP-autoimmunity connection through work
with a lupus model created by exposing mice to a hydrocarbon oil. Such
exposures cause normal mice to develop kidney disease, arthritis and other
conditions similar to lupus. However, mice with genetically disabled SAP
stayed healthy even after exposure.

To their surprise, researchers found that most immune system
functions appeared to be working normally in mice lacking SAP.

“We have identified other immune system proteins that
are potential targets for new autoimmune disease treatments, but they
all affect large portions of the immune system, making weakened immune
function a potential side effect of any new drug,” Peng explains.
“Our early experiments suggest targeting SAP for treatment may avoid
that risk.”

Peng cautions that errors in any one gene are unlikely to
be the sole cause of an acute autoimmune disorder like lupus. “It’s
very clear now that no single gene or even couple of genes are sufficient
to explain lupus,” he notes. “You probably need multiple malfunctions
in different genes to cause such a severe autoimmune syndrome.”

The multiple causes of lupus are likely reflected in the
multiple mouse models of the disorder, says Peng.

“Each of the animal models has slightly different
clinical aspects to it, probably because they represent a slightly different
facet of the human disease,” he explains. “It’s therefore
going to be very interesting to test if these are findings that can apply
to lupus generally or if they’re limited to subsets of lupus.”

Peng’s group recently identified another leash protein
from the same family of genes as Foxj1. They are currently working in
the lab to further understand the activity of all the proteins and also
have begun studying human lupus patients to see if they can detect signs
of abnormal activity in these proteins.